Thermoforming tubular articles

ABSTRACT

Tubular thermoformed articles are manufactured from tubular blanks which are heated to the annealing temperature for a time sufficient to anneal the articles by flowing hot liquid therethrough whilst constrained in the desire shape or form followed by flowing cold liquid therethrough to cool to set before being released from the constraint.

The invention relates to thermoforming articles and is more particularlyconcerned with methods of manufacturing articles having a predeterminedshape or form and manufactured from heat mouldable tubular material.

It is well known that thermoplastic tubular material can be bent with abending die at suitable temperatures followed by cooling to set thematerial in the bent configuration. Such a process suffers thedisadvantage that the finished articles are lacking in thermalstability, i.e. when the service conditions result in the article beingreheated to temperatures approaching the temperature at which they wereinitially bent there is a tendency for articles to resume the originalshape or form of the tubular material.

In general there have been two principal ways of manufacturing thermallystable predetermined shaped articles from tubular material that can bethermoformed. The first of these methods comprises injection moulding inwhich the article is formed in an openable mould. For a hollow articlethe mould comprises a casing and a core, the mould being designed sothat the core can be removed from the moulded article. At some stage insuch moulding, the mould is heated. In the other way of manufacture, thearticle is shaped from material while the material is in a plastic prpliable state and then the shaped article is baked or at least heated inan autoclave or oven.

Each of the above principal ways of manufacture have substantialdisadvantages. Thus, the cost of injection moulding apparatus issufficiently high to justify such expenditure only when very largenumbers of articles are to be manufactured. Further it is really onlyeconomical and practical to mould relatively small articles. This isequally relevant to a manufacturer using an autoclave since the cost ofa large volume autoclave can only be justified either for largeproduction volumes or for high value large articles.

Neither of the conventional manufactures is particularly suitable forlow volume production of relatively large awkward shaped articles. Thepresent invention addresses this difficulty.

According to the present invention there is provided a method ofannealing tubular articles of heat mouldable material comprising heatingthe articles to the annealing temperature for a time to effect theannealing thereof and cooling the articles to set by passing a fluidtherethrough while the articles are restrained in the predeterminedshape or form.

On releasing the constraint on the finished article it remains in thepredetermined shape or form and exhibits a high degree of thermalstability.

The invention also provides a method of manufacturing an article havinga predetermined shape or form from a tube of heat mouldable materialcomprising bending the tube to the predetermined shape, heating the tubeto the annealing temperature of the material by passing heated fluidthrough the tube, maintaining the flow of heated fluid for a period oftime sufficient to anneal the tube and cooling the tube while stillconstrained in the bent shape.

Preferably the tube is cooled by flowing cooling fluid through the tubeafter the flow of heating fluid is terminated. Preferably both fluidsare the same substance.

When the tube is not sufficiently flexible at ambient temperature to bereadily bent to the predetermined shape or form a heated fluid can beflowed through the tube to heat the tube before bending to a temperatureat which the tube is flexible enough to be fitted to a jig structuredefining the predetermined shape or form. The fluid which is flowedthrough the tube must have a boiling point higher than the desiredprocess temperature and can conveniently be oil.

Suitable heat mouldable or thermoformable materials are polyethylene,vinyl polymers such as polyvinyl chloride, polyvinyl acetate orpolyamides.

The tubular blank can, for example, comprise a tube and can be shapedeither by winding it onto a former to form a coil or by threading itonto or fixing it to a jig or like support structure.

Two examples of ways of performing the invention will now be describedwith reference to the accompanying drawings, in which:

FIG. 1 is a diagrammatic drawing of apparatus for carrying out a firstembodiment of the invention; and

FIG. 2 is a diagrammatic drawing of an article manufactured in a secondembodiment.

The manufacture to be described with reference to FIG. 1 is usable toform a coil of tubular plastics material which coils are usable forcoupling vacuum from a motor tractor to its trailer.

The initial tube is straight and flexible and is wound into a coil 11 ona rigid, in this example wooden, cylindrical former 12. The tube isformed of nylon hose, having an internal diameter of about 1 cm and awall thickness of about 0.1 cms. The former is about 12 to 15 cms. indiameter and about 60-75 cms long. The former is detachably androtatably mounted in a suitable frame (not shown). The tube is wound asa coil onto the former either while the former is detached from itsmounting frame or by rotating the former in its frame and feeding thetube onto the rotating former. The former may be helically grooved toreceive the tube. The rigidity of the wall of the tube is such that thebore remains substantially constant and the tube does not flatten.

When the coil has been wound on the former it is connected by a feedconduit 13 to a liquid pump 14 connected via a three-way valve V1 to ahot oil reservoir 16. The oil in the reservoir is Shell Clavus 25. Animmersion heater is used in this example to heat the oil to atemperature of 150° C. Oil is returned to the reservoir 16 by a returnconduit 17 including a second three-way valve V2. A second inlet of thesupply valve V1 is connected to the outlet of a cold oil reservoir 21and a second outlet of the return valve V2 is connected to the inlet ofthe reservoir 21. The cold oil is at room temperature but can be cooledto a lower temperature.

After winding the coil and connecting up the conduits, the pump isswitched on and hot oil flowed through the coil. The flow is maintainedfor about four minutes with the reservoir oil temperature at about 150°C. The valves V1 and V2 are then adjusted to change the oil flow fromhot oil to cold oil. The cold oil is allowed to flow for about fourminutes. The oil is then vented from the coil by setting the valve 14 toits third setting in which air is pumped into the coil. Finally, and ifrequired, the oil in the coil can be scavenged, for example by admittinga blast of nitrogen from a high pressure source into the coil.

After cooling, and scavenging if desired, the former is removed from itsmounting frame and taken out from the coil. The coil retains its helicalform. The helical coil can be extended and compressed in the manner of aconcertina. Suitable fittings can be swaged or otherwise secured to theends of the coil tube to permit it to be connected in a fluid or vacuumcircuit. The coil is thermally stable.

In the second embodiment the article is of relatively large size. Thearticle is usable, for example, in the air conditioning system of avehicle and replaces a similar product which has hitherto often beenassembled from discrete parts, at least some of which were metal. Thearticle is formed from what is commercially identified as "high pressurenylon hose". Such hose is flexible in that it can be wound intorelatively large diameter coils of, say for a 1.25 cm bore hose, 75-100cms diameter. However, when cold or at room temperature, the hose cannotbe taken around small radius curves e.g. curves of say 5 to 10 cmsradius.

As illustrated in FIG. 2, the article comprises five consecutivesections 31 to 35 and is between 11/2 and 2 meters in length. Thesections are not all in the same plane and, in fact, in this exampleonly sections 31, 32 are in the same plane. The article is formed usinga jig structure which provides a grooved path into which the hose can befitted. This is done by first laying out the hose alongside the jig pathand then flowing oil at a temperature of 100° C. through the hose fortwo minutes. By this time the hose has reached a temperature at whichthe nylon wall is sufficiently flexible to permit the tube to be fittedinto and secured in the jig path. The oil temperature is then raised tobetween 150° C. and 170° C. and caused to flow for a further fourminutes. The hose is connected in an oil flow circuit similar to thatillustrated in FIG. 1. Thereafter, the oil flow is changed from hot oilto cold (room temperature) oil to cool the hose to set and then ventedand, if required, scavenged.

Many changes may be made to the two ways hereinbefore described ofperforming the invention. For example, a fluid other than oil, eg a gasor water, may be passed through the tubular material. The article can bemoulded to have substantially any reasonable, required shape or form.The two described articles retain the full bore through their lengthsince collapse of the tube is prevented by the internal liquid pressure.However, if it is required to introduce a constriction this can be donewhen the tube is heated after bending. Again, if an exterior fixing flatis required this can be moulded into the tube wall. In both the examplesthe ambient medium was atmospheric air. However there could becircumstances where a different medium, for example a liquid, isrequired.

I claim:
 1. A method of manufacturing an open ended tubular article ofpredetermined shape or form from a tube of heat mouldable plasticsmaterial comprising preheating a tube to a forming temperature at whichit can be bent to a predetermined shape by passing a heated fluidtherethrough, bending the tube to the predetermined shape, heating thetube above the temperature at which it is bent to the annealingtemperature of the material by passing heated fluid therethrough indirect contact with the interior of the tube, maintaining the flow ofheated fluid for a period of time sufficient to anneal the tube andcooling the tube while constrained in the predetermined shape by passinga cooling fluid therethrough, the heating and cooling fluids comprisingthe same substance.
 2. A method according to claim 1 in which the tubeis high pressure nylon hose which is preheated by passing oil at 100° C.therethrough for a period of two minutes prior to bending, and annealedby passing oil at a temperature of between 150° C. and 170° C.therethrough for a period of four minutes after bending.